Electronic nightvision devices, where a scene is projected by a fast objective lens on the photo-cathode of an image intensifier and where a phosphorous screen of the same intensifier is viewed through a magnifier or a microscope, are used in the dark, with the night sky and the stars as the only illumination.
Such an instrument may have one eyepiece or it may be binocular, whereby the light from the phosporous screen is divided by a beam splitter to two eyepieces so that the observer may watch the phosphorous screen with both eyes. The device may be designed for handheld use--usually with several times magnification--or it may be fitted to the head like spectacles or attached to a helmet. In this latter application the device is called night visiion goggles, usually with unit magnification so that the observer is viewing the night scene in natural scale.
Since the observer in many applications must carry the night vision goggles for extended time periods, it is particularly important that they be as light as possible and that they not protrude unnecessarily in front of the observer, because the associated downward torque would cause neck strain. However, the components of the goggles objective lens, image intensifier tube, microscope unit, batteries etc.--are by necessity comparatively heavy and are according to prevailing design principles, so oriented that the whole device protrudes about 12-20 cm in front of the eyes of the observer. The experience with such an instrument is often that the front weight is embarrassing: Furthest away from the observer is a fast and thus comparatively heavy objective lens, followed by an image intensifier and finally a binocular microscope system with a beam splitter closest to the observer. The optical axis through the objective lens and the image intensifier are thus parallel with the line of sight against the scene being viewed.